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battery tester using LED?

I have been wondering, is there a way to make a simple battery tester?

Sure, I can always use multimeter to get an accurate result for "what is the voltage of my battery" or "is this battery still fresh? or not so fresh?"

But wouldn't it be nice to be able to just pop in and get some kind of feedback like: good - fresh, okay, or depleted.

What I have in mind is like a battery holder, that when you pop in a battery, it will light up led either red, yellow or green color depending on the battery voltage. Since most people use alkaline, how about 1.5 volts for the fresh voltage reference?

the tester probably have step up circuit to convert the V to the appropriate LED voltage. The problem is to make the circuit to step up voltage that is dependant on the voltage of the battery.
That requirement alone rule out simple step up circuit like the joule thief.

Upon looking for such circuit, i came upon a very simple step up circuit based on a chip PR4401. The output current is dependant on the input voltage (check the curve under 10 uH inductance) in the V range 1 - 1.5V, so maybe this is a good candidate for the stepping up circuit. I was thinking of putting a green led in paralel with red led + resistor. So as the current first starting up, the V drop only enough to light up the red led. but as the Vin increases, the V drop would be enough to light up the green led. Is this a good approach? or this idea is just stupid?

In any case, I am looking forward for any input/idea that you can provide.
thanks in advance

btw, the PR4401 seems to be only available in large quatities (3000)
It looks to be a very simple circuit, for all other intents and purposes, at least in driving a low power led (20 mA) from a single AA. It can do multiple leds in series too!!!! Anyone knows how to get it in small quantities (much less than 3000 )

Re: battery tester using LED?

You dont really need a step up circuit. Use a separate 9v battery to
power the LED. Since the 'tester' would only be used to quickly test
batteries, the 9v battery will last for years.

A good way to test a battery is to load it with a fixed resistor.
The resistor has to be appropriate for the kind of battery...for example...

The resistor for testing a 9v alkie battery might be 100 ohms, while
for testing a 1.5v cell could be 10 ohms, while to test a car battery
you would need 0.25 ohms very high power.
BTW i happen to have a car battery tester and the resistor is like
a toaster element, and gets just as hot during a test, and takes up
about 2x4x6 inches of space!

Re: battery tester using LED?

hi MrAl,

I am confused now.
For the 9V battery that powers the LED, i assume i need to put resistance in series so as to drive the led in very minimum current, say 1 mA.
Assuming I would like to test 1.5V alki battery, how would I connect the 10 ohms resistor? And how would the LED lights up from that?

Re: battery tester using LED?

Hi there,

The 'load' resistor always goes across the battery terminals (+) and (-).
You then measure the voltage of the battery with any means you wish.
For example, using three comparators (one IC package) and a few
resistors and a voltage reference IC you could have three different
LED's light up to indicate battery state...low, med, high.